1. The Field of the Invention
The present invention is directed generally to methods and apparatus for scribing and breaking substrates, and more specifically to methods and apparatus for scribing and breaking microsheets of glass.
2. The Relevant Technology
Thin sheets of glass, typically called "microsheets," consist of a ribbon of glass with a uniform thickness over the central region and small beads of thicker material at the edges. The edges provide some rigidity for handling but must be cut off before use of the microsheet.
One important use of microsheets is as covers for satellite solar cells. Microsheets suitable for this purpose typically comprise a borosilicate composition doped with cerium. In addition, such microsheets are commonly coated with single or multilayer thin films to effectuate certain thermal or optical characteristics. For example, in order to create a surface that reflects ultraviolet and infrared light wavelengths, microsheets for use with satellite solar cell covers are often coated with thin films of Ta.sub.2 O.sub.5 and SiO.sub.2. These thin films help control the thermal properties of the satellite solar cell covers, thus preventing the solar cells from heating up excessively.
Microsheets of glass used with satellite solar cell covers must be cut to fit the particular shape of the satellite solar cell. Such microsheets commonly range from about 50 microns to about 150 microns in thickness. In order to secure an appropriate fit, the microsheets are typically scribed and then broken along the scribe line. The critical depth for a scribe in such a microsheet via conventional techniques is from about 15% to about 20% of the microsheet's thickness. Hence, the critical scribe depth will generally fall in the narrow range from about 8 to about 30 microns. Even minor deviations in scribe depth will result in scribes that do not break properly. This is particularly problematic because the glass tends to break elsewhere and not along the scribe line if the scribe is too shallow, and if the scribe is too deep, the resulting edges are so rough that they render the microsheet useless.
The conventional material utilized for scribing purposes is a fresh, natural diamond. During use, the diamond cutting surface undergoes wear, causing its scribing capabilities to deteriorate. In order to achieve an adequate scribe depth, the pressure on the diamond scribe must be ever-increasing to counteract the wearing of the diamond.
Unfortunately, it is not a simple matter to discern the wear on the scribing diamond. Instead, the degradation of the diamond demands an operator to physically shut down the system and individually inspect each diamond, which results in increased costs and production delays due to the inspection time. All too often, the inspection is inadequate or untimely, resulting in undesirable glass breakage due to ineffective scribe depth.
Furthermore, because the wear on the diamond is difficult to ascertain and control, it is particularly difficult to obtain and maintain the critical scribe depth. Cuts made by the diamond scribe must be monitored very closely, again resulting in increased expense.
In addition, as the diamond degrades, the scribed edges of the microsheet decline in quality. Cracks tend to propagate along the rough, diamond-chipped edges and the microsheets tend to weaken. As a result, the microsheet may break during the process of bonding it on the solar cell.
Diamond scribes confront additional difficulties when faced with scribing microsheets coated with certain reflective thin films. For example, scribing through red-reflective layers, which are typically three times thicker than other reflective layers, tends to be problematic for diamond scribes. The stress in the thicker red-reflective layers makes the microsheet more difficult to cut and results in a drastically reduced yield of such coated microsheets when diamond scribes are utilized.